Figure 5: a) Schematic illustration of water intake
mechanism of dry NS and evaporation on the wet NS structure, Inset
showing transportation of water via natural evaporation and gravity
through the Micro/Nanochannels. b) Open Circuit Voltage
(Voc) and Short Circuit Current density
(Jsc) of four NSs. c) Long term reliability analyses of
the WS-WEG for 1 week d) Typical Voc and
Isc vs time for WS. e) Dry and wet WS as discharged and
charged stage. f) Histogram of the Voc for 30 WS-WEG
samples.
The resulting G-NS-G structure exhibits an open circuit voltage
(Voc) above 500 mV and a short circuit current density
(Jsc) of over 0.1 µA/mm2 with a small
amount of 1 ml drop DI water. The maximum current density
(Jsc) was achieved by the swift transportation of
electrons along the complete electrical circuit. The streaming
potential, explained later in the mechanism section, shows the positive
polarity towards the flow directions of the DI water. Therefore, only
the positive polarity is considered throughout this study. The
Voc and Isc show stable performance for
a couple of hours because of the constant evaporation of water into the
air. The orientation of these micro/nano-channels is placed parallelly
to the direction of gravity and capillary induced water flow.
Among these four NSs, the WS displayed the most efficient
Jsc, exceeding 0.20 μA/mm2, and
Voc, surpassing 600 mV. Multiple factors influence the
output Voc and Jsc of these four NSs.
Distribution of the micro/nanochannels on the shell structure, porosity
and their surface charge are the key parameters that affect the output
Voc and Jsc. Larger channel diameter, as
seen in AS, shows low performance compared to WS, which can be explained
by the equation of streaming potential. Because of the irregular channel
structure and pore distributions, FS and PS also underperform compared
with WS. Also, the asymmetrical-shaped narrow channels heighten
hydrodynamic resistance. The highest evaporation rate of WS was observed
in Figure 4 (c) also contributes to its superior electric
performance.
WS exhibits superior uniformity, compact, and consistent channel
structures among these four NSs, portrayed by large, interconnected 3D
-puzzle cell, facilitating more efficient fluid flow than
others.[28,30,31] This simple G-WS-G device can
generate a stable Voc of 612 mV and Jscof 0.15 µA/mm2 for a long duration. It consistently
maintained an open circuit voltage above 550 mV and short circuit
current above 17 µA (with 12 mm X 12 mm) for over a week without any
substantial fluctuations, as seen in Figure 5 (c). This
signifies the long-term stability of the nutshell-based WEG devices.
Replenishing the DI water every few hours at 0.3 ml/h maintained the
evaporation process under 25% humidity and 25 ºC with full surface
exposure. Because of the continuous evaporation of water, three layers
are created on the shell structure: wet, dry, and partially wet. The
partially wet regions present the highest contribution of streaming
voltage.[49]
Figure 5 (d) demonstrates the progressive rise in
Voc caused by the ongoing evaporation of DI water
through the micro/nanochannels. The slower mobility of DI water leads to
a prolonged duration for reaching the maximum voltage. A faster
achievement of the stable state was feasible because of the simultaneous
actions of gravity and capillary action. The Voc reaches
its maximum level while the specific saturation level is reached, and it
remains constant. Is decays because of the emission and
readjustment of electrons and reaches a stable condition after a while.
The NSs can continually harvest energy for a long time under wet
conditions, however, the entirely dry NS doesn’t have the capability of
generating energy. Therefore, the wetted NS can be considered as charged
whereas the fully arid NS can be considered as discharged, that is
depicted in Figure 5 (e). This nutshell-based WEG can
be reused cyclically by drying the soaked NS and re-wetting it for
energy harvesting.
Testing performed with platinum electrode (Figure-S 6 )
eliminates the possibilities of chemical reactions caused by the
electrode contaminations. Output voltage and current values might vary
slightly because of sample variations and measurement conditions. A
total of 30 independent samples were inspected to confirm the
reliability of the WS-WEG device (Figure 5 (f) ). The
findings signify that most samples demonstrated the revealed
Voc in the range of 580 mV to 620 mV.
Influential Factors Exploration on
Device
Performance
Multiple factors can influence the functionality of these WS-WEG devices
during actual usage. Hence, a systematic investigation was carried out
on several aspects, including relative humidity, temperature, different
concentrations of NaCl solutions, heights of the shell structure, and
polarity shifting of the device. The WS-WEG device was observed by
placing it on different levels of water heights. It is observed inFigure 6 (a) that a partially submerged device, one end is
exposed to air, sustains a consistent voltage due to water evaporation
and the streaming potential effect. However, once the device becomes
completely immersed, voltage reduces due to the inhibition of water
evaporation, the major mechanism for electrical production. The water
evaporation rate decreased across the micro/nanochannels for the fully
submerged device since the channels of the fully submersed device were
partially impeded. Capillary flow along the channels is less prominent
in fully submerged devices. Therefore, these devices show lower voltage
compared with the partially submerged ones. Upon removal from the water
reservoir, the device maintained in generating a low amount of voltage
for a limited duration due to the evaporation of residual water content
until it was completely dried.